The present invention generally relates to extrusions and, more particularly, to curved extrusions that are free of residual stress and to a method for forming a curved extrusion with reduced forming and stresses.
The requirements for material used in the aerospace industry are numerous. Demands include improved toughness, lower weight, as well as increased resistance to fatigue and corrosion. The boundaries of material properties are being constantly extended as manufacturers strive to give the next generation of aircraft improved performance while making them more efficient. Titanium and its alloys are increasingly used in the aerospace industry because of their excellent combination of high specific strength (strength-to-weight ratio), which may be maintained at elevated temperature, their fracture resistant characteristics, and their exceptional resistance to corrosion. The titanium alloy currently most commonly used is the alpha-beta alloy Ti6 Al4V. This conventional fine grain titanium alloy commonly used in section sizes up to 200 mm and may be used up to approximately 750° F. Ti6 Al4V is used to manufacture many aerospace airframe and engine components, such as blades, discs, rings, fasteners, cases, vessels, hubs, forgings, and T-chords. Despite the increased usage and production of titanium and its alloys, they are expensive when compared to many other metals and alloys, for example, aluminum and its alloys, because of the complexity of the extraction process, difficulty of melting, and problems during fabrication and machining. Therefore, near net-shape methods, such as extrusions, castings, isothermal forging, and powder metallurgy, have been introduced to reduce the cost of manufacturing titanium components.
The metal working process known as extrusion generally involves pressing metal stock, such as an ingot or billet, through a die opening matching the desired shape in order to form a product having indefinite length and a substantially constant cross section. Extrusion produces compressive and shear forces in the stock. Since no tensile stress is produced, the high deformation is possible without tearing the metal. The term extrusion is usually applied to both the process and the product obtained. A near net-shaped product may be obtained through extrusion, which is especially desirable in costly and difficult to machine alloys, such as alloys of titanium, steel, and nickel. Furthermore, extrusions generally have low tooling costs. However, disadvantages of extrusions include that extrusions have generally a constant cross-section and that extrusions are straight (as shown in FIGS. 1a and 1b). FIG. 1a provides a cross-sectional view of a typical prior art straight extrusion 10 shown in FIG. 1b. Extrusions may be formed to contour as a finishing operation, but the forming process may induce high residual stress in the extrusion and may result in loose tolerances, which are both undesirable. High residual stress of the extrusion may further result in distortion during machining. Furthermore, it might be desirable for some applications to locally change the cross-section of the extrusion.
For instance, one application of a titanium alloy extrusions in the aerospace industry could be for T-chords of an aircraft. T-chords may be used in assembling the wing to the fuselage of an aircraft. T-chords could be manufactured very cost effectively as extrusions since the T-chords are needed at long lengths having a generally constant cross-section, and need to be able to carry heavy loads. However, T-chords have to be curved to follow the shape of an aircraft wing. Since the curving process would induce residual stress within large extrusions, such as the T-chord, distortion of the extrusion may become a problem for assembling the T-chord. Presently, the problem is avoided either by cutting the extrusion in smaller pieces to be assembled to the wing box or by increasing the thickness of the cross-section of the extrusions.
Other prior art methods to produce near net-shape titanium and titanium alloy products include deposition processes, such as Laser Additive Manufacturing™ (LAM) offered by AeroMet Corporation (Eden Prairie, Minn.) and Laser Engineered Net Shaping™ (LENS) developed by Sandia National Labs which is being commercialized by Optomec Design Corporation. Both technologies utilize laser powder forming where typically metal or ceramic powder materials are delivered directly into a melt pool created by a laser beam to form parts in layerwise fashion. The strength of these technologies lies in the ability to fabricate fully dense metal or metal alloy parts with good metallurgical properties at reasonable speeds. While a variety of materials can be used such as stainless steel, Inconel, copper, aluminum etc., reactive materials such as titanium and titanium alloys are of particular interest. LAM is a fabrication method, which can be used to manufacture metallic preforms directly from computer-generated 3 D drawings. In this manner, freestanding shapes may be generated without molds or dies. The advantage of LENS lies in its ability to generate components having overhanging structures that are fully dense. However, these technologies have the disadvantage that the number and size of the components formed is limited and that production of components is costly. Deposition processes, such as LAM and LENS are very much suitable, for example, to rapidly produce replacement titanium components for the aerospace industries rather than to produce components that are constantly needed in high numbers.
As can be seen, there is a need for extrusions that may be formed to contour without inducing residual stress. Furthermore, there is a need to make local changes to the cross-section of an extrusion, for example, for the purpose of adding strength. Also, there is a need to provide large contoured extrusions made out of titanium or titanium alloys that are free of residual stress with more design flexibility at lower costs, and with reduced lead times. Moreover, there is a need to provide a method for forming extrusions into curved shapes without inducing residual stress.
There has, therefore, arisen a need to provide curved extrusions that are free of residual stress. There has further arisen a need to provide large titanium and titanium alloy extrusions that may be formed to contour. There has still further arisen a need to provide a method for forming large extrusions without inducing residual stress. There has still further arisen a need to provide a method for local design changes of the cross-section of extrusions.